Recently, research and development relating to communication systems used for sending and receiving various types of information to and from a communication device implanted in a living body, such as a body area network are being carried out. An antenna used for a communication device implanted in a living body such as the human body or animal body is preferably as compact and thin as possible. Loss of electric waves is large in a living body. Therefore, an antenna used for a communication device implanted in a living body is desired to be capable of communicating with other communication devices even in such a medium as a living body causing large electric wave loss.
In order to downsize an antenna, there have been proposed antennas each equipped with an emitting electrode folded at one or more portions thereof (Refer to, for instance, Published Japanese Translation of PCT International Publication for Patent Application (Kohyo) No. 2006-505973, Published Japanese Translation of PCT International Publication for Patent Application (Kohyo) No. 2002-533001, Japanese Laid-Open Patent Publication No. 2001-53535 and Japanese Laid-Open Patent Publication No. 2006-74351). These antennas are, however, have not been implanted in a living body for use thereof. Therefore, with the antennas as described above, it is impossible to reduce negative effects caused by electric wave loss in a living body, and the antennas are not suitable for use together with communication devices implanted in a living body.
On the other hand, there have also been proposed antennas which are implanted in a living body (Refer to, for instance, Published Japanese Translation of PCT International Publication for Patent Application (Kohyo) No. 2012-514418 and J. Kim et. al., “Implanted Antennas Inside a Human Body: Simulations, Designs, and Characterizations”, IEEE MTT Trans, vol. 52, no. 8, pp. 1934-1943). In each of the antennas, in order to match the impedance of the antenna with that of a living body, an emitting electrode of the antenna is covered with a dielectric material. In the antennas, the emitting electrode is folded so as to be compact.